Exposure refers to a process of exposing and transferring the chip pattern on a mask onto a silicon wafer, which is one of the important processes during semiconductor manufacturing. An exposure process comprises several sub-processes, such as wafer loading/unloading, pre-alignment, alignment, level adjustment detection, exposure, etc. The apparatus for exposure process is called a lithography machine. A main part of a lithography machine is the wafer stage motion positioning system, whose operation efficiency greatly influences the throughput of the lithography machine.
Present high-resolution lithography machines are prone to adopt a dual wafer stage structure. As shown in FIG. 1, a lithography machine adopting dual stage structure comprises from the top down an illumination system 4, a reticle stage positioning system 3, a projection object lens system 2, a focus and level adjustment detecting system 5, an alignment system 6, two wafer stage positioning units 100a, 100b, a base 1, etc. Wafer loading/unloading, pre-alignment, alignment, level adjustment detection and other pre-processing steps are carried out on the wafer stage unit 100a, which can work simultaneously with the wafer stage unit 100b responsible for the exposure process.
A dual wafer stage structure is described in PCT patent application WO98/40791, in which, each wafer stage has two exchangeable displacement units and an object holder. The object holders are connected to the guide bars. During operation, several pre-processing steps before exposure process are carried out on a wafer at the pre-processing workstation, while the exposure process is carried out on another wafer at the exposure workstation. After finishing these two series of steps, the two wafer stages move to an intermediate position to exchange the two object holders, so that either object holder moves from its displacement unit to the other holder's displacement unit, thereby accomplishing the exchange of two wafer stages.
The above mentioned structure enables the simultaneous operation and the exchange of two wafer stages, however, it has the following disadvantages: when the two wafer stages are exchanging, during a short period of time, the wafer stages are under an unrestricted status, so that the positioning accuracy of the system is influenced; besides, there exists a risk of collision of the two wafer stages during the exchange process due to the overlapped moving area and the high running speed of the two stages. Any collision will lead to a serious result. Although software can be used to control and protect the wafer stages, collision will still be possible if the software goes wrong or the power is suddenly cut off.